The invention relates in general to a switch, and more particularly to a circuit breaker, possessing a break chamber with low gas compression. The invention can apply equally well to single contact movement circuit breakers and to dual contact movement circuit breakers. In particular, the way in which dual contact movement circuit breakers operate and the way in which low gas compression circuit breakers operate have both been known separately for many years, however combining them presents drawbacks that are explained below. For low gas compression circuit breakers, reference can be made in particular to French patent FR 2 696 274.
It is appropriate to recall that the principle of low compression means that gas is compressed in the break chamber only during part of the stroke of the contacts, and generally during less than 50% thereof. This first portion of circuit breaker opening corresponds to the contacts moving from the closed position to the beginning of blasting the arc that follows separation of the contacts. Gas compression is at its maximum at the moment the contacts separate, and the pressure falls quickly as the arc is blasted. The amount of energy required for opening the circuit breaker is thus reduced during the second portion of the stroke of the contacts.
The principle of dual contact movement has been applied for a long time (see French patent FR 2 491 675), since it consists merely in driving both contacts simultaneously in opposite directions, either at different speeds or at speeds that are equal, thereby providing displacement that is symmetrical about the closed position. Drive can be provided by a linkage or by a rack-and-pinion system. Compared with a single movement device, the advantage of a dual movement device is that it enables the time required for separating the contacts to be reduced without increasing moving contact speed. The time required to separate the contacts depends on their mean relative speed and on their overlap distance R. Thus, for a symmetrical dual movement device, the contact separation time is approximately halved compared with a single movement device, for identical overlap distance R and mean contact speed. In addition, on separation of the contacts, each contact has moved through a distance of only R/2 in the case of a symmetrical dual movement circuit breaker, whereas the moving contact has moved through a distance R in the case of a single movement circuit breaker. Finally, the reduced moving contact speed in a dual movement device presents a significant advantage in terms of total kinetic energy, and energy consumption can be reduced by about 50% (in outline, the total moving mass is doubled but the mean speed of the contacts is halved and as a result total kinetic energy is approximately halved).
However, dual contact movement does not lead only to advantages, particularly if it is associated with a break chamber having low gas compression. Because of the reduced displacement of the contacts, the length L of the compression chamber (the relative stroke of the piston in the compression chamber) is halved, and as a result the blast pressure is likewise halved.
It should also be recalled that in most low gas compression circuit breakers the piston in the break chamber is generally held fixed inside the case during the first portion of circuit breaker opening. It is the compression chamber which is secured to the contact carrying the blast nozzle and which moves towards the piston in order to compress the gas (see above-mentioned patent FR 2 696 274). For a single movement circuit breaker, this gives a compression length L equal to the stroke of the moving contact during the first stage of opening, i.e. likewise equal to the overlap distance R of the contacts. By way of simplification, it is assumed that the compression volume Vc is equal to Lxc3x97S where S is the section of the piston (the bore).
In comparison, in a dual contact movement circuit breaker the compression length L is equal to R/2. Thus, to obtain a compression volume Vc equivalent to that of a single movement circuit breaker without increasing the overlap distance R of the contacts, it is necessary to double the section S of the piston. That solution presents three kinds of drawback:
it makes it necessary to increase the diameter of the case, and thus the overall size of the case;
it makes it necessary to double the force required for compression purposes in order to obtain the same gas pressure; and
it amounts practically to doubling the mass of the moving elements, thereby nullifying the saving in kinetic energy consumption that is achieved by using dual movement.
In order to increase compression volume without increasing piston section, certain single movement devices enable a compression length L to be obtained that is longer than the contact overlap distance R, typically to lie in the range 1.1 R to 1.25 R. To this end, the piston is no longer stationary during the compression stage, but is moved a little inside the case towards the compression chamber by means of a linkage connected to the piston and to the contact carrying the compression chamber. An example of such a system is to be found in European patent EP 0 664 552. The piston is then said to execute reciprocating movement since it moves in one direction during the compression stage and in the opposite direction after the contacts have separated. This movement during the first stage of movement is equal to the distance Lxe2x88x92R and in known devices that represents only 10% to 20% of the length L of the compression volume.
When applied to a circuit breaker having dual contact movement, such a system with reciprocating piston movement can make it possible to obtain a compression length L that lies typically in the range 1.1 R/2 to 1.25 R/2 instead of having L equal to R/2 for a fixed piston. The blast pressure thus remains well below that obtained in an analogous single movement circuit breaker.
An object of the invention is to provide a solution which remedies those drawbacks, and capable of being applied to all types of circuit breaker having a break chamber with reduced gas compression, regardless of whether they use single or dual contact movement.
Specifically, the invention makes it possible in a circuit breaker having dual contact movement to combine the advantages of single contact movement circuit breakers with the advantages of dual movement circuit breakers while avoiding their drawbacks. In particular, the invention provides a device possessing the same compression length L as a single movement device for given contact overlap distance R. The invention also makes it possible to improve the performance of single contact movement circuit breakers. Devices known in the prior art can typically achieve values of L lying in the range R to 1.25 R. In comparison, a device proposed in the context of the present invention makes it possible to obtain L equal to 2 R at least.
To this end, the invention provides a gas blast switch possessing a break chamber with low gas compression, the switch comprising a first contact and a second contact, the first contact being movable along a longitudinal axis and being secured to the break chamber in which the gas is compressed by a piston, displacement means for displacing said piston being arranged so that its movement changes direction inside the case of the switch after the gas compression stage, wherein said displacement means comprise a telescopic link connected to said piston, and wherein the length of the displacement of said piston inside said case during said compression stage is not less than the length of the displacement of said first contact during said same compression stage.
In a first embodiment of the switch of the invention, said second contact is movable along said longitudinal axis in the opposite direction to said first contact.
In a particular embodiment of the switch of the invention, said piston is connected in alternation with the second contact and with the first contact during the operation of opening the switch.
In a particular embodiment of the switch of the invention, said piston is secured to the second moving contact by means of the telescopic link throughout the gas compression stage, and is separated therefrom after said first and second contacts have separated so as to become secured to said first contact. This form of connection makes it possible to obtain a compression length L equal to said distance R.
In a particular embodiment of the switch of the invention, the telescopic link is formed by a first cylinder extending the piston and surrounded by a second cylinder, the second cylinder being fixed to a peripheral link that is permanently secured to the moving second contact. Said telescopic link comprises a locking assembly that unlocks at the end of the gas compression stroke to allow the movement of the piston to change direction and to follow the movement of the first contact after the first and second contacts have separated.
In a particular embodiment of the switch of the invention, said locking assembly is constituted by balls disposed in openings formed in the first cylinder, said balls being engaged in internal peripheral grooves of the second cylinder during the gas compression stage in order to lock said telescopic link.